The electric motors which are used in machine tools and in various industrial installations are often controlled such that a predefined setpoint-value position is reached. In this context, a main control loop is implemented in the control of such electric motors in order to adjust an actual position of the moving part of the motor to a setpoint-value position. The difference in position is processed via an electronic control unit, which determines a setpoint value for the supply current. The supply of the motor is therefore designed to feed the motor (each of its phases) so as to generate a supply current that corresponds with the setpoint-value current. In order to ensure the precision of the intended movement, it is advisable to make sure that the actual current agrees with the setpoint current. This is accomplished via a measurement of the current at each supply phase (in order to clarify the presentation hereof, hereinafter only one phase is taken into account), and with a second control loop which adjusts the measured supply current to the setpoint current.
A schematic view of a conventional motor control is shown in FIG. 1. A phase 2 (R, L) is energized via a power amplifier 4, which itself is supplied via high voltage HV (for example, 300 volts). At its input, electronic control unit 6 receives a position signal SP with respect to the position control and a measuring signal IM* of the electric current in phase 2. If the electronic unit is in a low-voltage zone (zone LV) and phase 2 is in a high-voltage zone (zone HV), in the normal case, the current is measured via a sensor of the magnetic type which has a ferrite ring 8, that is disposed around a conductor, which forms phase 2, as well as via an electronic circuit 10 for processing the electrical signal which is generated by the element for detecting the magnetic flux present in the ferrite ring. Such a measuring device is isolated galvanically from the phase. Electronic circuit 10 is supplied by the low voltage and is located in zone LV. An element for the galvanic isolation, which is suitable for transmitting an analog signal, is not necessary. It is noteworthy that such an element is relatively complex, and that high transmission precision of an analog signal via a galvanic isolation can only be achieved with difficulty. For this reason, to measure the current, one may select a Hall sensor or a sensor of the “fluxgate” type, as described, for example, in U.S. Pat. No. 4,914,381. However, such magnetic sensors are especially sensitive with regard to ambient magnetic fields which produce errors in the measurement. The signal-to-noise ratio is disadvantageous with such magnetic sensors to the extent that the precision of the current measurement in the phase is thereby impaired.
Electronic circuit 10 of the sensor generates an analog measuring signal IM, which is converted into a digital signal IM* by an analog-to-digital converter 12 likewise situated in the low-voltage zone. Based on this measuring signal, electronic circuit 6 determines the value of control signal SC*, which preferably is generated by a digital-to-analog converter 14 located in zone LV, and subsequently with the aid of a galvanic separating device which is designed to transmit the analog signals and to establish a galvanic separation GS. Equivalent analog signal SC is amplified by power amplifier 4. As mentioned above, galvanic separating device 16 is relatively complex, and the precision of the transmission of analog signal SC via the device is not particularly high. An alternative solution envisaged is to apply to the power amplifier, a signal that is modulated via the pulse width and is generated directly by the galvanic separating device by keying and blanking. This presents a further problem concerning the precision of the control signal, such that the transmission of the modulated signal produces a parasitic signal which must be filtered in analog fashion at the input of the power amplifier. This filtering is simplified by the selection of an increased frequency for the transmission of the signal, but is to the detriment of the resolution of the modulation of the pulse width, which becomes inadequate in so far as explained in the introduction to the U.S. Pat. No. 7,692,465. In addition, the filtering may be reflected in phase 2 itself, if a power amplifier is used in the switching mode; however, this option is not compatible with the demands with regard to precision placed on the present practical application.
The control of the electric motor explained above raises precision problems, first of all, in measuring signal IM* generated by electronic control unit 6, and subsequently in control signal SC generated by the electrical supply of the motor.